Interference filter for non-zero angle of incidence spectroscopy

ABSTRACT

The present disclosure relates to thin film optical interference filters. The filters include a substrate and a plurality of alternating material layers deposited on the substrate. When operated at about 45° angle of incidence, the filters exhibit at least one of improved polarization splitting, edge steepness, bandpass bandwidth, and blocking, relative to conventional thin film interference filters.

This application claims priority to U.S. Provisional Application No.60/940,701, filed May 30, 2007, the contents of which are incorporatedherein by reference.

The present disclosure relates to optical thin-film interferencefilters, including filters suitable for use in non-zero angle ofincidence spectroscopy. The present disclosure also relates tospectroscopy systems including such filters, and methods for making suchfilters.

BACKGROUND OF THE DISCLOSURE

Thin film interference filters are important components in systems foroptical measurement and analysis, such as Raman spectroscopy andfluorescence microscopy. In particular, thin film interference filters,such as optical edge filters, notch filters, and/or laser line filters(LLF's), are advantageously used in such systems to block unwanted lightthat would otherwise constitute or generate spurious optical signals andswamp the signals to be detected and analyzed. Thus, failure orinadequate performance of these filters can be fatal to operation of asystem in which they are utilized.

In general, interference filters are wavelength-selective by virtue ofthe interference effects that take place between incident and reflectedwaves at boundaries between materials having different refractiveindices. This interference effect is exploited in interference filters,which typically include a dielectric stack composed of multiplealternating layers of two or more dielectric materials having differentrefractive indices. In the case of a filter which substantially reflectsat least one band of wavelengths and substantially transmits at least asecond band of wavelengths immediately adjacent to the first band, suchthat the filter enables separation of the two bands of wavelengths byredirecting the reflected band, the resulting filter is called a“dichroic beamsplitter,” or simply a “dichroic” filter.

In a typical interference filter, each of the respective layers of thefilter stack is very thin, e.g., having an optical thickness (physicalthickness times the refractive index of the layer) on the order of aquarter wavelength of light. These layers may be deposited on one ormore substrates (e.g., a glass substrate) and in various configurationsto provide one or more of long-wave-pass (also called long-pass),short-wave-pass (also called short-pass), band-pass, or band-rejectionfilter characteristics.

In the case of prior known edge filters, the filter is configured so asto exhibit a spectrum having a clearly defined edge, wherein unwantedlight having wavelengths above or, alternatively, below a chosen“transition” wavelength λ_(T) is blocked, whereas desired light istransmitted on the opposite side of λ_(T). Edge filters which transmitoptical wavelengths longer than λ_(T) are called long-wave-pass (LWP)filters, and those that transmit wavelengths shorter than λ_(T) areshort-wave-pass (SWP) filters.

FIGS. 1A and 1B schematically illustrate the spectral transmission ofidealized LWP and SWP filters, respectively. As shown in FIG. 1A, anidealized LWP filter blocks light with wavelengths below λ_(T), andtransmits wavelengths above λ_(T). Conversely, as shown in FIG. 1B, anidealized SWP filter transmits light with wavelengths below λ_(T), andblocks light with wavelength above λ_(T).

Edge steepness and the relative amount of transmitted light areimportant parameters in many filter applications. As shown in FIGS. 1Aand 1B, an idealized edge filter has a precise wavelength edgerepresented by a vertical line at λ_(T). As such, an idealized filterhas an “edge steepness” (i.e. a change in wavelength over a definedrange of transmission) of 0 at λ_(T). However, real edge filters changefrom blocking to transmission over a small but non-zero range ofwavelengths, with increasing values of edge steepness reflecting an edgethat is increasingly less steep. The transition of a real edge filter istherefore more accurately represented by a non-vertical but steeplysloped line at or near λ_(T). Similarly, while an ideal edge filtertransmits all light in the transmission region (transmission T=1), realfilters have some amount of transmission loss, invariably blocking asmall portion of the light to be transmitted (T<1).

As a result, the reported edge steepness of a real edge filter dependson the transmission range over which it is defined. Further, as will bediscussed below, conventional edge filters exhibit polarizationsplitting when operated at a non-zero angle of incidence, in which casethe corresponding spectra for s and p-polarized light may not have thesame edge steepness.

Edge filters, notch filters, and laser line filters are particularlyuseful in optical measurement and analysis systems that use light from alight source, such as a laser, to excite/illuminate a sample at onewavelength λ_(L) (or a small band of wavelengths) and measure or view anoptical response of the excited sample at other wavelengths. Theexcitation light λ_(L) is delivered to the sample by an excitation lightpath, and the optical response of the sample is delivered to the eye ormeasuring instrument by a collection path. Notch filters are generallyspecialized implementations of edge filters, in that they exhibit a longwave edge and a short wave edge bordering a narrow region of lowtransmission. Laser line filters are generally configured so as totransmit as much light from a desired wavelength as possible, whileblocking other wavelengths.

These filters have been used to block spurious or unwanted light fromthe excitation and collection paths of an optical system. In the case ofedge filters, filters having higher edge steepness (i.e., a smallerdifference in wavelength over a defined transmission range) are capableof more effectively blocking spurious or unwanted light signals.Further, edge filters having lower transmission loss, if placed in thecollection path, are capable of passing more light from the sample tothe measuring instrument. Similarly, LLF's having lower transmissionloss, if placed in the excitation path, are capable of passing moreexcitation light from the light source (e.g., a laser) to the sample.

Raman spectroscopy is one example of an optical analysis system thatadvantageously employs dichroic/interference filters. In Ramanspectroscopy, molecular material is irradiated with excitation light,i.e., high intensity light of a given wavelength λ_(L) (or range ofwavelengths) Upon irradiation, the molecular material scatters theexcitation light. A small portion of the scattered excitation light is“Raman shifted,” i.e., it is shifted in wavelength above and/or belowλ_(L). This Raman shifting is attributed to the interaction of the lightwith resonant molecular structures within the material, and the spectraldistribution of the Raman shifted light provides a spectral“fingerprint” characteristic of the composition of the material.However, the bulk portion of the scattered excitation light is “Rayleighscattered,” i.e., it is scattered without a shift in wavelength.

Because the amount of Raman shifted light is very small relative to theamount of Rayleigh scattered light, it is necessary to filter theRaleigh scattered light from the collection path before it reaches thedetector. Without such filtering, the Rayleigh scattered light willswamp the detector, and may excite spurious Raman scattering in thecollection path. Filtering of the Rayleigh scattered light can beaccomplished, for example, by placing an edge filter, such as a LWPfilter having a transition wavelength λ_(T) just above λ_(L) (or rangeof wavelengths) between the sample and the detector. In this position,the LWP filter ensures that the light reaching the detector ispredominantly long-wavelength Raman-shifted light from the sample.Similar arrangements using edge filters can be used to analyze shortwavelength Raman-shifted light.

In an ideal Raman spectroscopy setup, a filter, such as a notch or edgefilter, is configured such that it blocks 100% of light having awavelength λ_(L) (or range of wavelengths) from reaching the detector,while allowing desired light to be passed to the detector formeasurement. This could be accomplished for example, if the filters wereconfigured so as to exhibit an ideal stopband that blocks 100% of lighthaving a wavelength λ_(L) (or range of wavelengths).

Conventional filters, however, exhibit narrow blocking or transmissionbands that exhibit a level of transmission and/or blocking that is lessthan optimum. The “blocking” of a filter at a wavelength or over aregion of wavelengths is typically measured in optical density (“OD”where OD=−log₁₀(T), T being transmission of the filter at a particularwavelength). Conventional filters that achieve high OD values at certainwavelengths or wavelength regions may not necessarily also achieve hightransmission (in excess of 50%, for example) at any other wavelengths orwavelength regions. High OD is generally exhibited in a fundamental“stopband” wavelength region, and such stopbands have associated withthem higher-order harmonic stopband regions occurring at otherwavelength regions.

These higher-order stopbands are one reason why it is difficult toachieve high transmission at wavelengths shorter than those over whichhigh blocking occurs. A stopband is a range of wavelengths over whichtransmitted light is strongly attenuated (T≦10%) due to constructiveinterference of the many partial waves of light reflected off of astructure with a periodic or nearly periodic variation of the index ofrefraction, as found in a thin-film interference filter. For a “quarterwavelength stack” structure comprised of alternating layers of high- andlow-index materials, each of which is approximately one quarter of aparticular wavelength λ₀ thick (in the material), the “fundamental”stopband is roughly centered on λ₀ and ranges from approximatelyλ₀/(1+x) to λ₀/(1−x), where x is related to the high and low index ofrefraction values, n_(H) and n_(L), respectively, according to

$x = {\frac{2}{\pi}{{\arcsin \left( \frac{n_{H} - n_{L}}{n_{H} + n_{L}} \right)}.}}$

If the layer-to-layer index of refraction variation is not a purelysinusoidal variation, but rather changes abruptly, as is typically thecase in a multi-layer thin-film interference filter, higher-orderstopbands exist at shorter wavelengths. For example, a quarter-wavestack having such abrupt refractive index changes exhibits“odd-harmonic” stopbands that occur approximately at the wavelengthsλ₀/3, λ₀/5, etc., and that range from approximately λ₀/(3+x) toλ₀/(3−x), for the third-order stopband, λ₀/(5+x) to λ₀/(5−x), for thefifth-order stopband, and so on. If the layers are not exactly aquarter-wave thick, there may also be “even-harmonic” stopbands thatoccur approximately at the wavelengths λ₀/2, λ₀/4, etc.

In general, known filters achieve high blocking over a wide range byutilizing a fundamental stopband, by combining multiple fundamentalstopbands, or by “chirping” (gradually varying) the layers associatedwith one or more fundamental stopbands. Regardless of the approach, thehigher-order harmonic stopbands associated with these blocking layersinhibit transmission at wavelengths shorter than the fundamentalstopband or stopbands.

FIG. 2 schematically illustrates a Raman spectroscopy system 10 having astandard configuration. As shown, this standard configuration includes alight source 1, such as a laser, an excitation filter 2, a sample 3, acollection filter 4, and a detector 5. In operation, light source 1emits light having a wavelength λ_(L) (or range of wavelengths) whichpasses though excitation filter 2 and illuminates sample 3 directly.Sample 3 scatters Raman shifted and unshifted excitation (Rayleighscattered) light. Collection filter 4 is positioned between sample 3 anddetector 5, such as a spectrophotometer. Collection filter 4 isconfigured to block the Rayleigh scattered light from sample 3 buttransmit as much of the Raman shifted light as possible, and as close toλ_(L) as possible.

In focusing or imaging systems that utilize high numerical aperture(high-NA) collection optics, however, it is desirable for light from thelight source and the collected signal light to share a common path. Tomeet this requirement, a two-filter solution is ideal. FIG. 3schematically illustrates a Raman spectroscopy system 20 having such aconfiguration.

As shown, this configuration generally includes a light source 11, suchas a laser, an excitation filter 12, a sample 13, a collection filter14, a detector 15, such as a spectrophotometer, and a beamsplitteroptical filter 16. Beamsplitter optical filter 16 is oriented atnon-zero angle of incidence, e.g., about 45°, relative to light incidentfrom light source 11, and is configured to reflect incident light fromlight source 11 onto sample 13, while transmitting Raman scattered lightreturning from Sample 13. Collection filter 14 is used in conjunctionwith beamsplitter optical filter 16 to ensure complete blocking ofincident light that is Rayleigh scattered or reflected from sample 13.Due to the orientation of beamsplitter optical filter 16 relative tolight from light source 11, the system shown in FIG. 3 is configured forso called, “non-zero angle of incidence” spectroscopy.

Increasing the angle of incidence of a traditional interference filterfrom normal generally affects the spectrum of the filter in two ways.First, the features of the filter spectrum are shifted to shorterwavelengths. And second, as the angle of the filter is further increasedfrom normal, the filter spectrum exhibits progressively increasing“polarization splitting.” That is, the filter produces two distinctspectra, one for s-polarized light, and one for p-polarized light. Therelative difference between the s and p spectra at a given point isgenerally called “polarization splitting.”

To illustrate this principal, reference is made to FIGS. 4A and 4B whichare plots of polarization splitting vs. angle of incidence for a quarterwave stack based on SiO₂ and Ta₂O₅ centered at 500 nm. In the plot ofFIG. 4A, the bandwidths of the stopbands associated with light of spolarization and p polarization are shown, with the bandwidths measuredin so-called “g-space.” The parameter g=λ₀/λ is inversely proportionalto wavelength and therefore directly proportional to optical frequency,and equals 1 at the wavelength λ₀ which is at the center of afundamental stopband associated with a stack of thin film layers eachequal to λ₀/4 n in thickness, where n is the index of refraction of eachlayer. The bandwidth in g-space is therefore equal to the differencebetween λ₀/λ_(S) and λ₀/λ_(L), where λ_(S) and λ_(L) are theshort-wavelength and long-wavelength edges of the stopband,respectively. The polarization splitting in g-space is thus simply onehalf of the difference between the bandwidths in g-space for s-polarizedand p-polarized light. As shown in FIG. 4B, the stack exhibitspolarization splitting of about 0.04 g-numbers when operated at 45° AOI.Increasing AOI to 60° results in polarization splitting of almost 0.08g-numbers. Decreasing AOI to 20° results in polarization splitting ofless than 0.02 g-numbers.

Many uses for thin film interference filters are known. For example,U.S. Pat. No. 7,068,430, which is incorporated herein by reference,discusses the use of such filters in Fluorescence spectroscopy and otherquantification techniques.

Dichroic optical filters have been proposed for use in optical systemsemploying a two filter design, such as the one shown in FIG. 3. However,as described above and shown in FIG. 4, traditional dichroic filtersexhibit substantial polarization splitting, particularly when operatedat about 45° Angle of incidence. This polarization splitting arises fromthe particular construction of a dichroic filter. As mentionedpreviously, traditional dichroic filters are generally made up ofalternating thin material layers having differing refractive index. Inaddition to the refractive index of each layer being different than thatof an adjacent layer, the effective refractive indices of eachindividual layer differ with respect to different polarizations oflight. That is, the effective refractive index for a layer is differentfor p-polarized light than it is for s-polarized light. As a result,s-polarized and p-polarized light are shifted to different degrees uponpassing through each layer in a dichroic filter. This difference inshift ultimately offsets the filter spectra corresponding to thesediffering polarizations, resulting in polarization splitting.

If a traditional dichroic filter is based on the first order stopband ofan angle-matched quarter-wave stack, estimating the polarizationsplitting between the stopband bandwidths of the filter is relativelystraightforward. That is, assuming the dichroic filter is made up of twomaterials having indices of n_(H) and n_(L), respectively, at 45° angleof incidence, the effective indices can be calculated as follows:

$\begin{matrix}{n_{L}^{S} = \sqrt{n_{L}^{2} - {\sin^{2}({AOI})}}} & (1) \\{n_{H}^{S} = \sqrt{n_{H}^{2} - {\sin^{2}({AOI})}}} & (2) \\{n_{L}^{P} = \frac{n_{L}^{2}}{\sqrt{n_{L}^{2} - {\sin^{2}({AOI})}}}} & (3) \\{n_{H}^{P} = \frac{n_{H}^{2}}{\sqrt{n_{H}^{2} - {\sin^{2}({AOI})}}}} & (4)\end{matrix}$

Wherein:

-   -   AOI is the incident angle in air, which is assumed to the        incident medium;    -   n_(L) ^(P) and n_(L) ^(S) are the effective refractive index of        the low index material in the dichroic stack for p-polarized        light and s-polarized light, respectively; \    -   n_(H) ^(P) and n_(H) ^(S) are the effective refractive index of        the high index material in the dichroic stack for p-polarized        light and s-polarized light, respectively; and    -   n_(H) ² and n_(H) ^(S) are the squares of the high and low        refractive indexes, respectively, associated with the two        materials, and which are independent of polarization.

The bandwidths and polarization splitting of the first-order stopbandfor the two polarizations may then be calculated as follows:

$\begin{matrix}{{\Delta \; g^{S}} = {\frac{4}{\pi}{\sin^{- 1}\left( \frac{n_{H}^{S} - n_{L}^{S}}{n_{H}^{S} + n_{L}^{S}} \right)}}} & (5) \\{{\Delta \; g^{P}} = {\frac{4}{\pi}{\sin^{- 1}\left( \frac{n_{H}^{P} - n_{L}^{P}}{n_{H}^{P} + n_{L}^{P}} \right)}}} & (6) \\{{PS}_{g} = \frac{{\Delta \; g^{S}} - {\Delta \; g^{P}}}{2}} & (7)\end{matrix}$

Wherein:

-   -   Δg^(S) and Δg^(P) are the bandwidths of the first order        (fundamental) stopband for s-polarized light and p-polarized        light, respectively, in g-space; and    -   PS_(g) is the polarization splitting for the first-order        stopband in g-space.        Alternatively, the polarization splitting may be expressed in        terms of wavelength. For example,

$\begin{matrix}{{PS}_{\lambda} = {\frac{\lambda_{0}}{1 - {\Delta \; {g^{S}/2}}} - \frac{\lambda_{0}}{1 - {\Delta \; {g^{P}/2}}}}} & (8)\end{matrix}$

wherein:

-   -   PS_(λ) is the polarization splitting of the long-wavelength edge        of the fundamental stopband (the edge associated with a        long-pass filter).        Often this value is expressed as a dimensionless value by taking        its ratio to the average wavelength of the edges associated with        s- and p-polarizations and expressing it as a percentage.

Polarization splitting has been utilized to design polarizing filterswhere high transmission and blocking are achieved for s and ppolarizations, respectively, over a defined wavelength band. However, inthe context of edge filters and beamsplitter optical filters,polarization splitting severely limits the edge steepness of lighthaving average polarization. Thus, it is desirable to minimizepolarization splitting as much as possible.

Several ways have been proposed to minimize polarization splitting. Forexample, one method proposed by Thelen (See A. Thelen, “Design ofOptical Interference Coatings,” McGraw Hill, 1989) utilizes tuningspacers of a multi-cavity Fabry-Perot bandpass filter to align the edgesof spectrum of s and p-polarized light. However, this method hassignificant limitations when used to create dichroic filters.

In Thelen's method, the starting layer structure is that of amulti-cavity Fabry-Perot bandpass filter with spacer layers havingoptical thickness equal to multiple half-waves of the referencewavelength used to define the associated stopband. In addition, the edgeof the resulting dichroic must be essentially at the center of theassociated stopband. This is unlike the filters according to the presentdisclosure discussed below, which differ from Thelen's approach both inlayer structure and placement of the dichroic edge with respect to thestopband. Indeed, as discussed below, filters according to the presentdisclosure do not contain the spacer layers required by Thelen'sapproach, and the dichroic edge may be placed virtually anywhere withrespect to the location of the stopband.

In addition, it has been shown that decreasing stopband bandwidth canresult in a corresponding decrease in polarization splitting. In thecase of a filter having a second order stopband, the bandwidth of thestopband is proportional to the material mismatch in the dielectricstack making up the filter, where “mismatch” refers to the deviation ofthe layer thicknesses from one quarter of a wavelength, while keepingthe sum of the thicknesses of each pair of high- and low-index layersequal to approximately one half of a wavelength. The greater themismatch, the higher the degree of polarization splitting, and viceversa. Thus, it has been shown that polarization splitting can beminimized by utilizing different (e.g., higher-order) stopbands andadjusting material mismatch in the dielectric stack making up a dichroicfilter.

However, while this method is effective, small mismatch always resultsin a filter having a narrow blocking region and lower blocking level,which is often not acceptable. Enhancement of the blocking region can beachieved, but only by increasing the number of layers in the dielectricstack. As a result, the performance of a traditional dichroic filterbased on a second order stopband is typically limited by the maximumcoating thickness allowed by the manufacturing process.

In addition, dual notch dichroic beamsplitters have been proposed foruse in optical systems having dual filter designs. FIG. 5 is a measuredspectrum of unpolarized light passing through an exemplary dual notchdichroic beamsplitter. As shown, this filter exhibits two narrowstopband regions 62 and 64 separated by a passband region having verynarrow bandwidth 66. The spectrum also exhibits a relatively narrowbandpass region 68 between stopband region 64 and a fundamental stopbandabove about 750 nm (not shown)

While prior known interference filters are useful for many applications,they generally exhibit unsatisfactory characteristics when operated atabout 45° angle of incidence. For example, the dual notch filter shownin FIG. 5 exhibits polarization splitting of 0.58% at one edge ofstopband 62, and 0.4% at one edge of stopband 64. However, this filterexhibits a relative passband bandwidth of only about 30%, which isunsatisfactory. The relative passband bandwidth is the ratio of thedifference between the long-wavelength of the passband and the dichroicedge wavelength to the dichroic edge wavelength (for a long-passdichroic filter). Further, this filter exhibits relatively poor edgesteepness of 1.26% at one edge of stopband 62, and 0.79% at one edge ofstopband 64, which are insufficient for many applications. The edgesteepness here is defined as the normalized wavelength differencebetween 10% and 90% transmission wavelengths for average polarizedlight.

Finally, angle matched notch filters have also been proposed for use innon-zero angle of incidence spectroscopy. Notch filters are described indetail in U.S. Pat. No. 7,123,416, the contents of which areincorporated herein by reference. However, when these filters areoperated at about 45° Angle of incidence, they suffer from significantpolarization splitting, as shown in FIG. 6 (where 72, 74, and 76correspond to the s spectrum, p spectrum, and average spectrum,respectively) F and described above. Accordingly, these types of filtersexhibit significant limitations when used in many optical measurementtechniques.

Thus, there is a need for improved interference filters that, whenoperated at about 45° angle of incidence, exhibit substantially improvedproperties relative to prior known filters. In particular, there is adesire in the art for improved interference filters that, when operatedat about 45° angle of incidence, exhibit at least one of improvedpolarization splitting, passband bandwidth, edge steepness, andblocking, relative to prior known filters.

SUMMARY OF THE DISCLOSURE

The present disclosure provides optical interference filters that aresuitable, for example, for use in Raman spectroscopy, fluorescenceimaging, and/or quantification applications. Among other things, thesefilters exhibit substantially better performance characteristics whenoperated at about 45° angle of incidence, relative to prior knowninterference filters. In particular, the present disclosure providesoptical filters that exhibit at least one of improved polarizationsplitting, edge steepness, passband bandwidth, and blocking, relative toprior known interference filters.

Consistent with the present disclosure are optical filters that includea substrate and a plurality of alternating first and second materiallayers on the substrate. The alternating first and second materiallayers have respectively different refractive indices. For the purposesof this disclosure, this structure is referred to as the “basicstructure.”

As will be discussed at length below, the plurality of alternatingmaterial layers of filters in accordance with the present disclosure maybe configured so as to achieve one or more of a variety of desiredoptical characteristics. In some embodiments, these layers areconfigured so as to obtain filters that exhibit, when light impinges onthe filter at about 45° angle of incidence, at least one spectrum havinga first stopband region and a second stopband region separated by apassband region. Thus, filters in accordance with the present disclosuremay, for example, exhibit a first spectrum for p-polarized light, asecond spectrum for s-polarized light, and an average spectrumcorresponding to the average of the s and p spectra.

In addition, the plurality of first and second material layers can beconfigured such that the first stopband region correlates to afundamental stopband of the filter, whereas the second stopbandcorrelates to a harmonic of the first stopband region, or a non-harmonicof the first stopband region, such as a passband defect. A non-harmonicstopband is a stopband that occurs in one of the passband regions oneither side of a fundamental stopband, and does not occur at awavelength which is an odd or even harmonic of the fundamental stopband.A non-harmonic stopband may be created by optimizing the thicknesses ofthe nearly quarter-wavelength-thick layers which form the fundamentalstopband in such a way as to cause the optical interference of light inthe layer structure to exhibit strong reflection over a region withinone passband, while exhibiting high transmission with relatively lowripple over the remaining portion of the passband. Hence, when formedthis way, this type of stopband is referred to here as a “passbanddefect.”

Further, the plurality of first and second material layers in filters inaccordance with the present disclosure may be configured so as tooptimize one or more characteristics of the filter spectrum. Forexample, the plurality of layers may be configured so as to optimize atleast one of polarization splitting, edge steepness, blocking, andpassband bandwidth exhibited by the filter spectrum, particularly whenthe filter is operated at about 45° angle of incidence. In someembodiments, filters in accordance with the disclosure may be configuredso as to optimize two or more of these features relative to one another.Moreover, these filters may be configured so as to optimize at least oneof the aforementioned characteristics in at least one region of thefilter spectrum, such as at the edge or base of a stopband region.

The present disclosure also describes methods of making the opticalfilters described herein, as well as systems using the optical filtersdescribed herein. Thus, consistent with the present disclosure areoptical filters having the structure described herein, and which areproduced by known deposition techniques, such computer controlled ionbeam sputtering.

Also consistent with the present disclosure are optical systems thatincorporate at least one of the filters described herein as an opticalfilter. For example, these systems may include the filters describedherein as an edge, laser line, or dichroic beamsplitter filter fornon-zero angle of incidence spectroscopy. Of course, the filtersdescribed herein may also be used in other systems and in other waysconsistent with the use of previously known optical filters.

Additional objects and advantages of the disclosure will be set forth inpart in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the disclosure. Theobjects and advantages of the disclosure will be realized and attainedby means of the elements and combinations particularly pointed out inthe appended claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the disclosure, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages, nature, and various additional features of thedisclosure will appear more fully upon consideration of the illustrativeembodiments described below in detail in connection with theaccompanying drawings. In the drawings:

FIG. 1A is a spectrum of an idealized long wave pass interferencefilter.

FIG. 1B is a spectrum of an idealized short wave pass interferencefilter.

FIG. 2 is a schematic illustration of a Raman spectroscopy system havinga standard configuration

FIG. 3. is a schematic illustration of a Raman Spectroscopy Systemhaving a two-filter configuration.

FIG. 4A is a plot of bandwidth in g-space vs. angle of incidence for atraditional quarter wave stack based on SiO₂ and Ta₂O₅ centered at 500nm.

FIG. 4B is a plot of polarization splitting vs. angle of incidence for atraditional quarter wave stack based on SiO₂ and Ta₂O₅ centered at 500nm.

FIG. 5 is a spectrum of light of average polarization passing through adual notch dichroic beamsplitter.

FIG. 6 is a design spectrum of an angle matched notch filter measured at45° angle of incidence

FIG. 7 is a calculated spectrum of a minimal polarization splittingdichroic filter in accordance with the present disclosure.

FIG. 8 is a calculated spectrum of a minimal polarization splittingdichroic filter in accordance with the present disclosure.

FIG. 9 is a comparison of a dichroic design in accordance with thepresent disclosure and a traditional angle matched short wave passfilter having comparable coating thickness.

FIG. 10 is a design spectrum of a 532 nm steep dichroic beamsplitter inaccordance with the present disclosure.

FIG. 11 is a magnified portion of a design spectrum of a 532 nm steepdichroic beamsplitter in accordance with the present disclosure.

FIG. 12 is a magnified portion of a measured spectrum of a 532 nm steepdichroic beamsplitter in accordance with the present disclosure.

FIG. 13 is a simulation of a measured spectrum of a 532 nm steepdichroic beamsplitter, using a 2-degree cone-half angle beam at thefilter.

FIG. 14 is a design spectrum of a 785 nm steep dichroic beam splitter inaccordance with the present disclosure.

FIG. 15 is a magnified portion of a design spectrum of a 785 nm steepdichroic beam splitter in accordance with the present disclosure.

FIG. 16 is a measured spectrum of a deeply blocking, steep 532 nm edgefilter measured at 45° Angle of incidence.

FIG. 17. is a magnified portion of a design spectrum of a deeplyblocking, steep 532 nm edge filter measured at 45° Angle of incidence.

DETAILED DESCRIPTION

Reference will now be made in detail to various exemplary embodiments ofthe present disclosure, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

One aspect of the present disclosure relates to optical interferencefilters that exhibit improved characteristics when operated at about 45°Angle of incidence, relative to traditional optical interferencefilters.

As used herein, the term, “about 45° angle of incidence” means that thefilter in question is oriented such that light from a light sourceimpinges on a surface of the filter at an angle ranging from about 40°to about 50°, unless otherwise specified. Of course, the filtersaccording to the present disclosure may be operated at any other angleof incidence. For example, the filters according to the presentdisclosure may be operated at an angle of incidence chosen from about43° to about 48°, from about 44° to about 46°, and substantially 45°. Insome embodiments of the present disclosure, the filters described hereinare operated at substantially 45° angle of incidence.

All of the optical filters according to the present disclosure generallyinclude the same basic structure. That is, they generally include atransparent substrate and a plurality of alternating first and secondmaterial layers deposited on a surface of the substrate. The pluralityof alternating first and second material layers have respectivelydifferent refractive indices.

A wide variety of materials may be used to form the alternating firstand second material layers. Among such materials, non-limiting mentionis made of metals, metallic and non-metallic oxides, transparentpolymeric materials, and so called “soft” coatings, such as sodiumaluminum fluoride (Na₃AlF₆) and zinc sulfide (ZnS). Further non-limitingmention is made of metallic oxides chosen from SiO₂, Ta₂O₅, Nb₂O₅, HfO₂,TiO₂, and Al₂O₅. In some embodiments of the present disclosure, thefirst and second material layers are Nb₂O₅ and SiO₂, respectively

Filters in accordance with the present disclosure may be manufacturedusing deposition methods and techniques that are known in the art. Forexample, these filters may be made with a computer controlled ion beamsputtering system, such as the one described in U.S. Pat. No. 7,068,430,which is incorporated herein by reference. In general, such a system iscapable of depositing a plurality of alternating material layers,wherein the thickness of each layer may be precisely controlled.

Further, filter designs in accordance with the present disclosure may beproduced by known thin-film filter design techniques. For example, thesefilter designs may be produced by optimizing the filter spectra andstructure of an initial design, such as a traditional short wave pass orlong wave pass interference filter against a target spectrum using knownoptical optimization routines. Non-limiting examples of suchoptimization routines include the variable-metric or simplex methodsimplemented in standard commercial thin-film design software packages,such as TFCalc by Software Spectra, Inc. and The Essential Macleod byThin Film Center, Inc. A detailed description of filter designtechniques that can be used to produce filter designs according to thepresent disclosure may be found in U.S. Pat. No. 7,068,430, which isincorporated herein by reference.

The filters of the present disclosure differ from traditionalinterference filters in that during production, the individualthicknesses of the alternating material layers making up theinterference stack are carefully controlled so as to achieve desiredoptical characteristics that are not exhibited by prior known opticalfilters. For example, optical filters consistent with the presentdisclosure may be configured so as to exhibit, when operated at about45° angle of incidence, at least one of improved polarization splitting,edge steepness, passband bandwidth, and blocking, relative to priorknown interference filters.

Accordingly, one aspect of the present disclosure relates tointerference filters having the basic structure described above, whereinthe alternating first and second material layers are configured suchthat when light from a light source impinges on the filter at an angleof incidence of about 45°, the filter defines a spectrum for s-polarizedlight, a spectrum for p-polarized light, and an average spectrumcorresponding to light having an average polarization (i.e.,corresponding to an average of the s and p spectra). Each of thesespectra includes a first stopband region and a second stopband regionseparated by a passband region.

As a non-limiting example of the basic spectral characteristics offilters according to the present disclosure, reference is made to FIGS.7 and 8, which plot transmittance vs. wavelength for exemplary dichroicfilters that are in accordance with the present disclosure, and whichexhibit minimal polarization splitting. As shown in both of these FIGS.,these exemplary filters exhibit a spectrum for s-polarized light, aspectrum for p-polarized light, and an average spectrum corresponding toan average of the s and p spectra. Each of these spectra exhibit apassband region 82, 92, a first stopband region 84, 94, and a secondstopband region 86, 96. Of course, the location and width of thestopband regions and the passband of the filters according to thepresent disclosure are not limited to those shown in these FIGS.

As used herein, the term “stopband region,” means a range of wavelengthsover which transmitted light is strongly attenuated (i.e., transmissionis ≦10%) due to interference of the many partial waves of lightreflected off of a structure with a periodic or nearly periodicvariation of the index of refraction, as found in a thin-filminterference filter. In the case of the filters described herein, lightthat is not transmitted is generally reflected, though blocking by othermeans (e.g., absorption) is also possible.

The first stopband region 84, 94 and second stopband regions 86, 96 maybe centered on any wavelength region of the electromagnetic spectrum, solong as they do not overlap with one another. For example, the first andsecond stopband regions may encompass distinct wavelength ranges in the200-1250 nm portion of the electromagnetic spectrum. In some embodimentsof the present disclosure, the first and second stopbands are present inthe 350-1250 nm range, such as the 350-850 nm range, more specificallythe 400-750 nm range.

The second stopband region, such as regions 86, 96, may encompasswavelengths that are shorter or longer than those encompassed by thefirst stopband region. In some embodiments, the second stopband isplaced so as to attenuate and/or block substantially monochromatic lightof a given wavelength. For example, the second stopband may be placed soas to block laser light having a wavelength within the 200-1250 nm rangeof the electromagnetic spectrum. In some non-limiting embodiments, thesecond stopband is placed so as to attenuate and/or block substantiallymonochromatic light having a wavelength chosen from 488.0 nm, 514.5 nm,532.0 nm, 623.8 nm, and 785.0 nm.

Placement of the cut-on edge and/or cut-off edge of the first and/orsecond stopband regions may be controlled by optimizing the layerthickness of the individual first and second material layers. Thus, forexample, filters in accordance with the disclosure may be configuredsuch that an edge wavelength (λ_(EW)) of at least one of the firstand/or second passband region is located at wavelengths that are 2% orless from incident monochromatic light having a wavelength λ_(L). Thatis, |λ_(EW)−λ_(L)|/λ_(L)*100% may be 2% or less. Of course, λ_(EW) maybe located closer to or farther from λ_(L), such as ≦1%, and ≦0.5%.

In some embodiments of the present disclosure, the plurality ofalternating material layers are configured such that the first stopbandregion corresponds to a fundamental stopband of the filter, and thesecond stopband region corresponds to a an odd or even harmonic stopbandof the fundamental stopband.

In other non-limiting embodiments, filters in accordance with thepresent disclosure may be configured such that the first stopband regioncorresponds to a fundamental stopband of the filter, and the secondstopband region corresponds to a non-harmonic stopband region, such as a“passband defect.” That is, in these embodiments, the second stopbandregion encompasses a range of wavelengths that do not correspond to anodd or even harmonic of the fundamental (first) stopband region. For amore specific description of fundamental and harmonic stopbands,reference is made to U.S. Pre-Grant Publication No. 2008-0037129, thecontents of which are incorporated herein by reference.

Filters with a passband defect may be created in a variety of ways. Asan example, a long-pass dichroic filter based on a passband defect tothe short-wavelength side of a fundamental stopband may be obtained froma starting structure correlating to a quarter-wave stack consisting ofplurality of alternating first and second material layers havingdifferent refractive indexes. The quarter-wave optical thickness isdefined with respect to a reference wavelength chosen such that theassociated fundamental stopband is above the desired transmittingwavelength region of the target LWP dichroic. The location of thepassband defect generally does not coincide with the higher harmonicstopbands associated with the fundamental stopband, and therefore occursin a region that is transmitting prior to any optimization of layerthickness. Once the starting structure is established, thin-film filteroptimization algorithms known in the art may be used to graduallyincrease the blocking level over the passband defect wavelength region,and then to optimize the layer structure after each increase, until thetarget blocking level is achieved.

Regardless of whether the second stopband region correlates to aharmonic or non-harmonic stopband of the first stopband region, it ispossible through careful control of the configuration of the pluralityof alternating first and second material layers to optimize variousaspects of the filter spectrum, as described below.

In some non-limiting embodiments, filters according to the presentdisclosure are configured so as to maximize edge steepness in a regionof the filter spectrum corresponding to at least one edge wavelength ofthe first and/or second stopband regions, particularly when the filteris operated at about 45° Angle of incidence. As used herein, and unlessotherwise specifically stated, the term “edge steepness,” refers to therelative difference (in percent) of the wavelength of a spectrum oflight having average polarization at the 10% transmission point (λ₁₀)and the 90% transmission point (λ₉₀) of a long or short wave edge of thefirst and/or second passband region, relative to a corresponding edgewavelength of the relevant stopband. That is, “edge steepness” (ES) isdefined by the expression:

ES=(|λ₉₀−λ₁₀|/λ_(EW))*100%

wherein λ_(EW) is the corresponding edge wavelength.

Further, as used herein, the term, “corresponding edge wavelength,”refers to the cut-on or cut-off frequency of the first or secondstopband region for light of average polarization corresponding to theparticular edge under consideration. Thus, for example, if the passbandregion of the filter spectrum encompasses wavelengths longer than thesecond stopband region such as shown in FIG. 7, λ_(EW) refers to thecut-on frequency 87 of the second stopband (the long wave edge of thesecond stopband), or the cut-off frequency 88 of the first stopband (theshort wave edge of the first stopband). The opposite is true if thepassband region encompasses wavelengths shorter than said secondstopband region. That is, in those cases, “edge wavelength” (λ_(EW))refers to the cut-off frequency of the second stopband (i.e., the shortwave edge of the second stopband) and the cut-on frequency of the firststopband (the long wave edge of the first stopband), for light ofaverage polarization.

According to some embodiments, filters consistent with the presentdisclosure have the basic structure described above, and define at leastone spectrum having the general features described above when the filteris operated at about 45° Angle of incidence. Moreover, these filters maybe configured such that at least one edge of the first and secondstopband regions exhibits an edge steepness ranging from ≦0.76%, ≦0.75%,≦0.65%, ≦0.50%, ≦0.40%≦0.25%, ≦0.23%, ≦0.17%, and ≦0.10% or less.

In non-limiting embodiments, filters consistent with the presentdisclosure are configured such that at least one edge of the secondstopband region has an edge steepness within the above described ranges,wherein the second stopband region correlates to a harmonic ornon-harmonic of the first stopband region. For example, the filters maybe configured such that the second stopband region of the filterspectrum exhibits an edge steepness within these ranges, and correlatesto a passband defect.

Also in accordance with the present disclosure are interference filtershaving the basic structure described above, and which exhibit, when thefilter is operated at about 45° angle of incidence, at least one filterspectrum having the general features described above. In thesenon-limiting embodiments, the filters are configured such that at leastone of the first and second stopband regions exhibit a defined edgesteepness between the optical density 5 and 90% transmission pointsand/or the optical density 3 and 90% transmission points of the filterspectrum for light of average polarization. For example, filters inaccordance with the present disclosure may be configured so as toexhibit an edge steepness, between the optical density 5 and 90%transmission points of least one of the first and second stopbandregions, that ranges from about ≦1.82%, ≦1.26%, ≦1.0%, ≦0.75%, ≦0.50%,and ≦0.46% or less, relative to a corresponding edge wavelength.Similarly these filters may be configured such that they exhibit an edgesteepness between the optical density 3 and 90% transmission pointsranging from about ≦0.76%, ≦0.75%, ≦0.56%, ≦0.50%, ≦0.25%, and ≦0.24% orless, relative to a corresponding edge wavelength.

In non-limiting embodiments, at least one edge of the second stopbandregion exhibits an edge steepness within these ranges, wherein thesecond stopband correlates to a harmonic or non-harmonic of the firststopband region.

In still other non-limiting embodiments, filters according to thepresent disclosure may be configured such that the first and secondstopband regions each comprise a long wave edge and a short wave edge.For example, as shown in FIG. 9, first stopband 102 and second stopband103 exhibit short wave edges 107 and 108 respectively, and long waveedges 109 and 110, respectively. Short wave edges 107, 108 have awavelength (or range of wavelengths) λ_(S1), and λ_(S2), respectively,and long wave edges 109, 110, have a wavelength (or range ofwavelengths) λ_(L1) and λ_(L2). At least one of long wave edges 109, 110and short wave edges 107, 108 have an edge steepness, relative to acorresponding edge wavelength, falling within the above describedranges. In a non-limiting embodiment, at least one of short wave edge107 and long wave edge 110 have an edge steepness falling within theabove described ranges.

The plurality of alternating material layers in the filters according tothe present disclosure may also be configured so as to optimizepolarization splitting exhibited by the filter spectrum.

Thus, consistent with the present disclosure are interference filtershaving the same basic structure and general spectral characteristicsdescribed above. The plurality of alternating material layers of thefilter are configured so as to minimize polarization splitting when thefilter is operated at about 45° Angle of incidence.

As used herein, the term, “polarization splitting” refers to thedifference (in percent) between a wavelength for s-polarized light(λ_(50S)) and a wavelength for p-polarized light (λ_(50P)), relative toan average of λ_(50S) and λ_(50P), wherein λ_(50S) and λ_(50P) aremeasured at a 50% transmission point of an edge of a stopband region ofthe corresponding s and p spectra. That is, polarization splitting (PS)at a given edge of a stopband is defined by the relation:

PS=(|λ_(50S)−λ_(50P))/[λ_(50S)+λ_(50P))/2]

In some embodiments, the filters according to the present disclosure areconfigured so as to exhibit, when operated at 45° angle of incidence, atleast one spectrum including a first stopband region and a secondstopband region separated by a passband region, wherein the first orsecond stopband region of the filter exhibits polarization splittingchosen from about ≦0.50%, ≦0.25%, ≦0.10%, ≦0.039%, ≦0.033%, and ≦0.015%or less. For example, filters according to the present disclosure may beconfigured such that the first stopband region correlates to afundamental stopband, the second stopband region correlates to aharmonic of the first stopband, and the second stopband region exhibitspolarization splitting within the above described ranges when the filteris operated at about 45° angle of incidence.

In other non-limiting embodiments, filters according to the presentdisclosure may be configured such that the second stopband regioncorrelates to a non-harmonic stopband (such as a passband defect) of thefirst stopband region, and the second stopband region exhibitspolarization splitting within the above described ranges when the filteris operated at about 45° angle of incidence.

Also consistent with the present disclosure are interference filtersthat have the same basic structure described above, and which exhibitboth low polarization splitting and wide passband bandwidth relative toa corresponding edge wavelength (λ_(EW)).

As used herein, the term, “passband bandwidth,” means the width (inpercent) of the passband region separating the first and second stopbandregions of the filter spectrum, relative to the corresponding edgewavelength λ_(EW) of the second stopband region. Thus, for example, if apassband is present between a longwave edge (λ_(LW2)) of the secondstopband region, and a shortwave edge (λ_(SW1)) of the first stopbandregion, the passband bandwidth (PB) is defined by the expression:

PB=(|λ_(SW1)−λ_(LW2)|/λ_(EW))*100%,

where λ_(EW) is defined as indicated above for the relevant edge of thesecond stopband region.

Thus, in some embodiments, filters in accordance with the presentdisclosure may exhibit, for example, ≦0.5% polarization splitting, suchas ≦0.25% polarization splitting, in conjunction with a defined passbandbandwidth. For example, filters in accordance with present disclosuremay be configured such they exhibit, when operated at about 45° Angle ofincidence, polarization splitting within the above described ranges inconjunction with a passband bandwidth ranging from about ≧58.85%,≧55.97%, ≧50.00%, ≧30.00%, ≧25.00%, ≧23.52%, ≧10.00%, ≧8.06%, ≧7.66%,and ≧5.00%. For example, filters according to the present disclosure maybe configured such that the second stopband region exhibits polarizationsplitting within the above described ranges, and correlates to aharmonic of the first stopband region or a non-harmonic of the firststopband region, such as a passband defect.

The achievable passband bandwidth in filters according to the presentdisclosure is dependent upon the edge steepness and/or polarizationsplitting. The steeper the edge or the smaller the polarizationsplitting, the more difficult it is to achieve a wider passband.

Also consistent with the present disclosure are interference filtershaving the same general structure described above, and which exhibitextended and/or enhanced blocking in a wavelength range corresponding toat least one of the first and second stopband regions when operated atabout 45° Angle of incidence.

Extended blocking about the first and/or second stopband regions may beaccomplished by adding additional layer structure to the plurality ofalternating material layers present in filters according to the presentdisclosure. The addition of extended blocking to a complex filtercoating is described in detail in U.S. Pat. No. 6,809,859, which isincorporated herein by reference. Similarly, blocking within the firstand or second stopband regions of filters according to the presentdisclosure may be enhanced by depositing additional alternating firstand second material layers.

In this way, filters according to the present disclosure may beconfigured to provide deep blocking of wavelengths within at least oneof the first and second stopband regions. That is, filters according tothe present disclosure may be configured so as to transmit about 10%,5%, 1%, or substantially 0% (i.e., optical density 6) of wavelengthsfalling within at least one of the first and second stopband regions. Innon-limiting embodiments, filters according to the present disclosureare configured such that the second stopband region exhibits blockingwithin the above described ranges, and correlates to a harmonic of thefirst stopband region or a non-harmonic of the first stopband region,such as a passband defect.

Filters according to the present disclosure can improve the performanceof a variety of optical analysis systems that illuminate/excite a samplewith light of a first wavelength (or range of wavelengths) to produce ameasurable or viewable response of light at a second wavelengthdifferent from the first. Such systems, which include Raman spectroscopyand fluorescence microscopy, typically have the typical constructionshown in FIG. 2, or the two-filter construction shown in FIG. 3.

Filters according to the present disclosure may be used in known opticalsystems in any manner consistent with the use of interference filtersknown in the art. For example, filters according to the presentdisclosure may be used in optical systems employing the two filterconfiguration shown in FIG. 3. As previously described, such a systemgenerally includes a light source 11, such as a laser, an excitationfilter 12, a sample 13, a collection filter 14, a detector 15, and abeamsplitter optical filter 16. Beamsplitter optical filter 16 isoriented at non-zero angle of incidence, e.g., about 45°, relative tolight incident from light source 11, and is configured to reflectincident light from light source 11 onto sample 13, while transmittingscattered light having a corresponding shift in wavelength (e.g., Ramanscattering) returning from Sample 13. Collection filter 14 is used inconjunction with beamsplitter optical filter 16 to ensure completeblocking of incident light that is Rayleigh scattered or reflected fromsample 13. Alternatively, beamsplitter optical filter 16 may itself havehigh blocking at the excitation wavelength, thus obviating the need forthe then redundant collection optical filter 14.

Filters in accordance with the present disclosure may be used, forexample, as beamsplitter optical filter 6 in optical systems of thegeneral two filter configuration shown in FIG. 3. In this case, thealternating first and second material layers may be configured such thatwhen light from the light source impinges on the dichroic beamsplitteroptical filter at an angle of incidence of about 45°, the filter definesa spectrum for s-polarized light and a spectrum for p-polarized light,with each spectra defining a first stopband region and a second stopbandregion separated by a passband region. The layers are also configuredsuch that the dichroic beamsplitter optical filter exhibits at least oneof improved polarization splitting, edge steepness, blocking, andpassband bandwidth, as described above.

Use of filters according to the present disclosure in such systemsallows signals to be measured closer to the wavelength or wavelengthregion associated with the excitation laser or source, while maintainingnecessary high blocking of the source light from the detection system.Thus, in Raman spectroscopy, filters according to the present disclosureallow the measurement of signals closer to the laser line. As a result,vibrational lines with very small energy shifts can be measured, thusproviding information about a measured sample that would otherwise beobscured by Rayleigh scattered light. In fluorescence spectroscopy andimaging, the ability to measure signals closer to the source wavelengthmeans that more signal can be captured, thus increasing the sensitivityof the system (ability to measure very small signals) and thespecificity of the system (decrease in background noise). Furthermore,filters according to the present disclosure that exhibit enhancedblocking may allow for one or more of the excitation and/or collectionfilters of the system shown in FIG. 3 to be removed.

The disclosure will be more fully illustrated using the followingnon-limiting examples.

Other than in the examples, or where otherwise indicated, all numbersexpressing endpoints of ranges, and so forth used in the specificationand claims are to be understood as being modified in all instances bythe term “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the specification and attached claimsare approximations that may vary depending upon the desired propertiessought to be obtained by the present disclosure. At the very least, andnot as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldbe construed in light of the number of significant digits and ordinaryrounding approaches.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the present disclosure are approximations, unlessotherwise indicated the numerical values set forth in the specificexamples are reported as precisely as possible. Any numerical value,however, inherently contains certain errors necessarily resulting fromthe standard deviation found in their respective testing measurements.

EXAMPLES Examples 1 and 2 Minimal Polarization Splitting, Long Wave PassSteep Edge Dichroic Filter Configurations Based on the Concept ofPassband Defect

A filter design corresponding to minimal polarization long wave passsteep edge dichroic filter based on the concept of passband defect wasproduced by optimizing a traditional dichroic short wave pass (SWP)filter spectrum and structure against a design spectrum using well-knownoptimization algorithms (e.g., the variable metric approach). That is,this design was optimized starting from a dichroic SWP filter comprisinga substrate and approximately 150 alternating quarter wavelength thicklayers of materials having high and low refractive index at a referencewavelength, respectively, and in view of a target (design) spectrumhaving desired spectral characteristics.

In the design spectrum, the edge of the SWP was chosen to be slightlylonger than a specified long wavelength edge of the dichroic passband.The passband ripple of the design spectrum was optimized. Afteroptimizing the passband ripple, the optimization continued while theblocking level just below the cut-on wavelength of the passband defectwas gradually increased.

The spectra of this filter design was calculated at 45° angle ofincidence. These calculated spectra are shown in FIG. 7. As shown, thisfilter design exhibits average spectrum 83, s-spectrum 84, p-spectrum85, a first stopband region 84 above about 560 nm, a second non-harmonicstopband region 86 ranging from about 450 to 508 nm, and a passbandregion 82 between the first and second stopband regions 84, 86.Moreover, the second stopband 86 region exhibits minimal polarizationsplitting (0.039%, or less than 0.2 nm) at its long wave edge 87.

The calculated s and p spectra of example filter 1 were compared to thespectra of a comparative filter (comparative example 1) that exhibitedonly a fundamental stopband, and no passband defect. This comparison isshown in FIG. 9. In this FIG., spectra 105 and 205 correlate to thespectra for s-polarized and p-polarized light, respectively, of examplefilter 1 (identified as “Filter 2” in the figure legend). Spectra 101and 201 correlate to the spectra for s-polarized and p-polarized light,respectively, of the comparative filter (identified as “Filter 1” in thefigure legend). As shown, example filter 1 exhibited spectralcharacteristics similar to those of the comparative filter, except thatthe filter of example 1 exhibited a passband defect to theshort-wavelength side of the fundamental stopband.

The calculated edge steepness and polarization splitting of the filterof example 1 was also compared to those of a commercially availablestandard dichroic filter known in the art, i.e., the filter manufacturedby Semrock, Inc. under part number FF506-Di02. The resulting data isreproduced in the table below.

TABLE 1 Comparison of a filter design in accordance with the presentdisclosure and a conventional dichroic filter design: Coating AveragePolarization Edge S-Pol P-Pol Polarization Thickness 2% 10% 90%Steepness 50% 50% Splitting Comparative 7.7 504.4 507.2 510.9 3.7 510.5508.6 1.9 example 1 example 1 12.1 506.9 507.7 508.8 1.1 508.4 508.30.2 * Edge Steepness is defined as the transition width between 10% and90% transmission levels.As shown, the filter of example 1 exhibited significantly better edgesteepness and polarization splitting, relative to comparative example 1.

A second filter design (example 2) similar to example filter 1 above wasalso produced. The calculated average spectrum for this design (FIG. 8)was compared to the spectrum of the standard angle-matched notch filtershown in FIG. 6. While the calculated spectra for the filter of example2 exhibited relatively limited bandwidth compared to that of the notchfilter, as shown in the following table, it achieves better edgesteepness for unpolarized light, and comparable edge steepness fors-polarized light with a coating thickness less than half that of thenotch filter.

TABLE 2 comparison of a second filter design in accordance with thepresent disclosure and a conventional notch filter: Coating AveragePolarization S Polarization Thickness (μ) OD 5 90% ES OD 5 90% ESExample 2 13.4 502.5 508.8 6.3 505.0 508.8 3.8 Comparative 31.5 534.4541.9 7.5 538.9 542.0 3.1 example 2 (notch filter) * Edge Steepness isdefined as the transition width between OD 5 and 90% transmission level

Examples 3 and 4 532 nm Steep Dichroic Beamsplitter

A first 532 nm dichroic beamsplitter optical filter (example 3) wasproduced having the configuration shown in the following table:

TABLE 3 Design structure of a 532 nm steep dichroic beamsplitter 532 nmSteep Dichroic Beamsplitter Coating Thickness (μm): 14.572006282 TotalLayers: 92 Layer Material Thickness (nm) 1 Nb2O5 15.76397 2 SiO2299.964436 3 Nb2O5 140.250246 4 SiO2 191.877075 5 Nb2O5 127.915529 6SiO2 182.984338 7 Nb2O5 127.387605 8 SiO2 179.120336 9 Nb2O5 127.71754210 SiO2 175.632888 11 Nb2O5 128.592224 12 SiO2 176.380586 13 Nb2O5128.373916 14 SiO2 178.887283 15 Nb2O5 127.769399 16 SiO2 184.194878 17Nb2O5 127.93426 18 SiO2 188.227308 19 Nb2O5 128.005756 20 SiO2188.057134 21 Nb2O5 127.918216 22 SiO2 184.475321 23 Nb2O5 128.072983 24SiO2 179.425546 25 Nb2O5 128.141557 26 SiO2 176.964355 27 Nb2O5127.96974 28 SiO2 177.506078 29 Nb2O5 126.511604 30 SiO2 178.33606 31Nb2O5 122.577018 32 SiO2 184.039011 33 Nb2O5 117.335837 34 SiO2194.189076 35 Nb2O5 112.228887 36 SiO2 203.60499 37 Nb2O5 106.362721 38SiO2 211.476307 39 Nb2O5 104.05433 40 SiO2 216.082874 41 Nb2O5 104.9778142 SiO2 216.022612 43 Nb2O5 105.579945 44 SiO2 215.384734 45 Nb2O5109.206235 46 SiO2 216.710917 47 Nb2O5 112.573883 48 SiO2 215.791662 49Nb2O5 111.467447 50 SiO2 214.359317 51 Nb2O5 112.161114 52 SiO2215.045267 53 Nb2O5 111.2683 54 SiO2 213.279207 55 Nb2O5 110.267674 56SiO2 214.458176 57 Nb2O5 111.006435 58 SiO2 215.231417 59 Nb2O5110.245668 60 SiO2 214.169518 61 Nb2O5 111.027028 62 SiO2 214.34487 63Nb2O5 111.673756 64 SiO2 214.244813 65 Nb2O5 111.079041 66 SiO2213.789538 67 Nb2O5 112.533959 68 SiO2 214.210094 69 Nb2O5 112.724502 70SiO2 214.10514 71 Nb2O5 111.816605 72 SiO2 214.903751 73 Nb2O5111.375223 74 SiO2 216.600697 75 Nb2O5 108.409546 76 SiO2 217.075285 77Nb2O5 105.631166 78 SiO2 219.15066 79 Nb2O5 104.675816 80 SiO2221.358226 81 Nb2O5 100.306396 82 SiO2 221.855185 83 Nb2O5 96.432497 84SiO2 219.84993 85 Nb2O5 101.481757 86 SiO2 217.432062 87 Nb2O5108.404809 88 SiO2 213.890199 89 Nb2O5 111.04977 90 SiO2 217.589309 91Nb2O5 106.544147 92 SiO2 114.923947

The design for this filter was produced by optimizing a standarddichroic filter comprising alternating quarter wavelength thick layersof SiO₂ and Nb₂O₅ against the target spectra shown in FIGS. 10 and 11.

The filter configuration was physically produced using a computercontrolled ion-beam deposition system, such as the one described in U.S.Pat. No. 7,068,430. The resulting filter was measured at 45° Angle ofincidence using a Perkin Elmer Lambda 900 spectrophotometer with a2-degree cone-half-angle beam at the filter. A magnified portion of themeasured spectra 131, 133, 135 is shown in FIG. 12. As shown, averagespectrum 133 exhibited a non-harmonic second stopband region having longwave edge at about 536 nm. This edge had an edge steepness (10%-90% T)for light of average polarization of 0.65%, relative to the edgewavelength. Furthermore, polarization splitting at the 50% transmissionpoint for this edge was 0.45%. The filter also exhibited a fundamentalstopband having a short wave edge around 850 nm (not shown in FIG. 12).The passband bandwidth was 58.9%, relative to the long wave edge of thenon-harmonic second stopband region.

Spectra 131, 133, and 135 shown in FIG. 12 did not precisely correlateto the design spectra shown in FIG. 11. To clarify this issue, simulatedspectra 141, 143, 145 (shown in FIG. 13) of a 532 nm steep dichroicbeamsplitter were plotted using a 2-degree cone-half-angle beam at thefilter. The simulated spectra of FIG. 13 closely correlated to themeasured spectra of FIG. 12. Thus, the simulated spectra demonstratedthat the disagreement between the measured spectra in FIG. 12 and thedesign spectra in FIG. 11 was dominated by the non-collimated nature ofthe spectrophotometer beam.

An additional 532 nm steep dichroic beamsplitter (example 4) wasproduced largely in accordance with the design of example 3, but had atotal coating thickness of about 14.3 μm. This filter was then comparedto the filter of example 3, so as to investigate the impact of coatingthickness on edge steepness. The data obtained is reproduced in thefollowing table.

TABLE 4 Dependence of coating thickness on edge steepness for 532 nmdichroic beam splitters: Thickness CHA Edge Steepness (μm) (Deg) 1% 2%5% 10% 50% 90% 1%-50% 2%-50% 5%-50% 10%-50% Ex. 3 14.6 0.00 531.77532.45 533.20 533.70 535.32 537.16 0.67% 00.54% 0.40% 0.29% Ex. 4 14.30.00 530.24 531.47 532.60 533.28 534.85 535.72 0.87% 0.63% 0.42% 0.29%

As shown, as coating thickness is increased, the value of edge steepnessdecreased over all indicated transmission ranges. That is, the 532 nmfilter having a coating thickness of about 14.6 μm exhibited better edgesteepness than a filter having a similar design having a coatingthickness of about 14.3 μm

Examples 5-7 785 nm Steep Dichroic Beamsplitter

A 785 nm dichroic beamsplitter optical filter design was produced havingthe configuration shown in the following table.

TABLE 5 structure of a 785 nm dichroic beamsplitter 785 nm SteepDichroic Beamsplitter Coating Thickness (μm): 22.027679258 Total Layers:102 Layer Material Thickness (nm) 1 Nb2O5 32.65508 2 SiO2 144.868199 3Nb2O5 47.804313 4 SiO2 128.394549 5 Nb2O5 238.282792 6 SiO2 278.529226 7Nb2O5 205.569093 8 SiO2 192.698961 9 Nb2O5 213.370004 10 SiO2 229.79832411 Nb2O5 202.31804 12 SiO2 229.540528 13 Nb2O5 188.952969 14 SiO2264.186477 15 Nb2O5 166.668797 16 SiO2 298.763611 17 Nb2O5 151.800937 18SiO2 306.540671 19 Nb2O5 153.743939 20 SiO2 292.519538 21 Nb2O5167.77654 22 SiO2 265.665013 23 Nb2O5 181.805452 24 SiO2 248.085954 25Nb2O5 186.587916 26 SiO2 257.083035 27 Nb2O5 173.731249 28 SiO2282.190763 29 Nb2O5 159.397771 30 SiO2 300.391953 31 Nb2O5 150.96526 32SiO2 297.936495 33 Nb2O5 162.276647 34 SiO2 278.171719 35 Nb2O5176.668889 36 SiO2 253.863045 37 Nb2O5 184.792068 38 SiO2 254.299535 39Nb2O5 177.746861 40 SiO2 272.711789 41 Nb2O5 163.373578 42 SiO2296.233849 43 Nb2O5 153.524699 44 SiO2 299.382853 45 Nb2O5 158.508489 46SiO2 281.140846 47 Nb2O5 174.034337 48 SiO2 258.219413 49 Nb2O5182.515237 50 SiO2 252.516009 51 Nb2O5 180.195999 52 SiO2 270.147819 53Nb2O5 165.079108 54 SiO2 292.635319 55 Nb2O5 154.092521 56 SiO2300.457145 57 Nb2O5 157.556848 58 SiO2 283.249826 59 Nb2O5 170.688922 60SiO2 263.521222 61 Nb2O5 182.754251 62 SiO2 250.831796 63 Nb2O5180.190883 64 SiO2 267.736417 65 Nb2O5 168.130035 66 SiO2 289.992505 67Nb2O5 154.816559 68 SiO2 301.48422 69 Nb2O5 156.252091 70 SiO2292.362158 71 Nb2O5 167.902499 72 SiO2 267.994834 73 Nb2O5 182.180612 74SiO2 256.094057 75 Nb2O5 185.837901 76 SiO2 256.023623 77 Nb2O5182.259064 78 SiO2 275.674377 79 Nb2O5 172.851921 80 SiO2 293.233127 81Nb2O5 167.346137 82 SiO2 309.467491 83 Nb2O5 166.563984 84 SiO2315.796872 85 Nb2O5 168.81202 86 SiO2 317.38589 87 Nb2O5 166.46106 88SiO2 318.205316 89 Nb2O5 166.733122 90 SiO2 321.459683 91 Nb2O5165.490765 92 SiO2 324.579443 93 Nb2O5 163.747759 94 SiO2 314.956531 95Nb2O5 162.072937 96 SiO2 322.030717 97 Nb2O5 168.63477 98 SiO2 359.0025299 Nb2O5 101.921796 100 SiO2 91.621751 101 Nb2O5 14.838899 102 SiO279.718824

Like examples 3 and 4 above, the design for the filter of example 5 wasproduced by optimizing a standard dichroic filter comprising alternatingquarter wavelength thick layers of SiO₂ and Nb₂O₅. However, in thiscase, the basic structure and spectra were optimized against the targetspectra 153, 161, 163, 165 shown in FIGS. 14 and 15.

The resulting filter exhibited spectral characteristics, when measuredat 45° Angle of incidence, largely consistent with the target spectra.That is, this filter exhibited a fundamental stopband region having ashort wave edge around 1240-1250 nm, and a non-harmonic second stopbandregion below about 780 nm. A bandpass region separated the fundamentalstopband region and the non-harmonic second stopband region. Thebandpass bandwidth was about 56%, relative to the long wave edge of thenon-harmonic second stopband region. the edge steepness at the long waveedge of the non-harmonic stopband was 0.24%. the edge steepness (10%-90%T) at the long wave edge of the non-harmonic stopband was 0.52%.

Further, like examples 3 and 4, multiple filters in accordance with thisdesign were designed having different overall coating thicknesses. Thedesign spectra were compared and the resulting data is provided in thefollowing table:

TABLE 6 Dependence of coating thickness on edge steepness for 785 nmdichroic beamsplitters: Thickness CHA Edge Steepness (μm) (Deg) 1% 2% 5%10% 50% 90% 1%-50% 2%-50% 5%-50% 10%-50% Ex. 5 22.0 0.00 784.33 786.40787.57 790.22 791.67 0.74% 0.48% 0.34% Ex. 6 21.5 0.00 783.33 785.65786.91 789.51 790.85 0.79% 0.49% 0.33% Ex. 7 28.4 0.00 784.32 785.65786.91 787.69 789.55 790.76 0.66% 0.50% 0.33% 0.24% 0.25 784.35 786.36787.53 790.21 791.74 0.74% 0.49% 0.34% 0.50 784.24 786.24 787.41 790.16791.93 0.75% 0.50% 0.35% 1.00 783.84 785.79 786.97 790.03 792.62 0.79%0.54% 0.39%

Example 7 Deeply Blocking, Steep 532 nm Beamsplitter Design

A design for a deeply blocking, steep 532 nm beamsplitter for 45° Angleof incidence spectroscopy was designed having the configuration shown inthe following table.

TABLE 7 design structure of a steep 532 nm beamsplitter for 45° Angle ofincidence: 532 nm Deeply Blocking Steep Beamsplitter Coating Thickness(μm): 28.441678135 Total Layers: 240 Layer Material Thickness (nm) 1Nb2O5 125.326398 2 SiO2 189.82584 3 Nb2O5 142.239469 4 SiO2 173.603106 5Nb2O5 117.551329 6 SiO2 133.761606 7 Nb2O5 80.048395 8 SiO2 176.142504 9Nb2O5 94.522896 10 SiO2 130.594691 11 Nb2O5 79.110445 12 SiO2 171.42398213 Nb2O5 95.099021 14 SiO2 130.08483 15 Nb2O5 78.427458 16 SiO2166.362706 17 Nb2O5 98.106453 18 SiO2 132.199068 19 Nb2O5 77.000752 20SiO2 157.603991 21 Nb2O5 101.809349 22 SiO2 136.519889 23 Nb2O576.525829 24 SiO2 149.287636 25 Nb2O5 103.878664 26 SiO2 141.700953 27Nb2O5 76.171685 28 SiO2 141.748625 29 Nb2O5 102.836029 30 SiO2151.235606 31 Nb2O5 76.671595 32 SiO2 135.237111 33 Nb2O5 99.608324 34SiO2 161.797651 35 Nb2O5 77.413062 36 SiO2 131.503309 37 Nb2O5 94.77140638 SiO2 168.571161 39 Nb2O5 80.958326 40 SiO2 128.724034 41 Nb2O588.765883 42 SiO2 172.451601 43 Nb2O5 85.879418 44 SiO2 128.608638 45Nb2O5 82.76891 46 SiO2 171.164311 47 Nb2O5 91.683182 48 SiO2 131.16936949 Nb2O5 78.174113 50 SiO2 165.223996 51 Nb2O5 96.979355 52 SiO2135.753497 53 Nb2O5 75.953529 54 SiO2 155.54757 55 Nb2O5 100.806536 56SiO2 142.76079 57 Nb2O5 74.918393 58 SiO2 145.729079 59 Nb2O5 101.12840160 SiO2 152.577714 61 Nb2O5 75.495573 62 SiO2 137.404314 63 Nb2O598.714134 64 SiO2 161.823972 65 Nb2O5 77.800849 66 SiO2 131.799092 67Nb2O5 93.62446 68 SiO2 169.469091 69 Nb2O5 81.286423 70 SiO2 129.76041271 Nb2O5 87.106824 72 SiO2 172.74435 73 Nb2O5 86.67709 74 SiO2129.628565 75 Nb2O5 81.980207 76 SiO2 169.63066 77 Nb2O5 93.161138 78SiO2 131.596083 79 Nb2O5 78.207451 80 SiO2 162.830265 81 Nb2O5 97.82903682 SiO2 138.108686 83 Nb2O5 75.326723 84 SiO2 153.591247 85 Nb2O5100.724734 86 SiO2 145.680217 87 Nb2O5 75.197663 88 SiO2 144.136845 89Nb2O5 100.023055 90 SiO2 155.996046 91 Nb2O5 75.797013 92 SiO2136.731146 93 Nb2O5 96.701027 94 SiO2 164.807958 95 Nb2O5 78.485102 96SiO2 131.486595 97 Nb2O5 91.416386 98 SiO2 170.843571 99 Nb2O5 82.948453100 SiO2 129.597724 101 Nb2O5 85.684493 102 SiO2 171.307995 103 Nb2O589.26102 104 SiO2 130.290103 105 Nb2O5 80.451223 106 SiO2 167.153794 107Nb2O5 95.259769 108 SiO2 133.387034 109 Nb2O5 77.191911 110 SiO2158.612514 111 Nb2O5 99.755764 112 SiO2 139.656216 113 Nb2O5 75.567149114 SiO2 149.125107 115 Nb2O5 101.280773 116 SiO2 148.680223 117 Nb2O575.567766 118 SiO2 140.751704 119 Nb2O5 99.493004 120 SiO2 158.91319 121Nb2O5 76.931062 122 SiO2 133.924406 123 Nb2O5 95.587146 124 SiO2167.046455 125 Nb2O5 79.941891 126 SiO2 130.595713 127 Nb2O5 90.061043128 SiO2 171.52332 129 Nb2O5 84.639663 130 SiO2 129.724731 131 Nb2O584.731083 132 SiO2 170.853256 133 Nb2O5 90.506855 134 SiO2 130.698407135 Nb2O5 80.919633 136 SiO2 166.148719 137 Nb2O5 95.948475 138 SiO2134.107378 139 Nb2O5 78.455937 140 SiO2 160.402121 141 Nb2O5 99.679602142 SiO2 138.661918 143 Nb2O5 77.568835 144 SiO2 155.399004 145 Nb2O5101.437398 146 SiO2 143.460187 147 Nb2O5 77.747731 148 SiO2 150.835494149 Nb2O5 102.425545 150 SiO2 147.617068 151 Nb2O5 77.514841 152 SiO2148.320077 153 Nb2O5 102.383455 154 SiO2 150.835187 155 Nb2O5 77.594384156 SiO2 144.567254 157 Nb2O5 101.963371 158 SiO2 153.995372 159 Nb2O577.581 160 SiO2 139.575503 161 Nb2O5 100.629357 162 SiO2 158.270796 163Nb2O5 78.23092 164 SiO2 134.943788 165 Nb2O5 97.362407 166 SiO2164.518246 167 Nb2O5 79.948805 168 SiO2 131.020116 169 Nb2O5 92.463462170 SiO2 169.687184 171 Nb2O5 83.612812 172 SiO2 129.311392 173 Nb2O586.405451 174 SiO2 171.732418 175 Nb2O5 88.583284 176 SiO2 129.57488 177Nb2O5 81.449042 178 SiO2 168.473041 179 Nb2O5 94.322516 180 SiO2131.891973 181 Nb2O5 77.64882 182 SiO2 160.980285 183 Nb2O5 99.082782184 SiO2 136.972706 185 Nb2O5 76.270067 186 SiO2 150.794779 187 Nb2O5101.82662 188 SiO2 144.810218 189 Nb2O5 76.247458 190 SiO2 142.197031191 Nb2O5 101.08276 192 SiO2 154.472747 193 Nb2O5 76.645516 194 SiO2135.588916 195 Nb2O5 97.089812 196 SiO2 164.201697 197 Nb2O5 78.884226198 SiO2 130.455866 199 Nb2O5 92.066688 200 SiO2 170.656157 201 Nb2O582.645702 202 SiO2 129.356456 203 Nb2O5 86.614591 204 SiO2 172.151688205 Nb2O5 88.107814 206 SiO2 129.509539 207 Nb2O5 81.813237 208 SiO2168.307689 209 Nb2O5 94.040778 210 SiO2 131.741612 211 Nb2O5 78.477373212 SiO2 161.070545 213 Nb2O5 99.303611 214 SiO2 135.909174 215 Nb2O576.189676 216 SiO2 153.895605 217 Nb2O5 101.524691 218 SiO2 142.414629219 Nb2O5 75.918059 220 SiO2 143.004075 221 Nb2O5 104.234156 222 SiO2148.478627 223 Nb2O5 76.261156 224 SiO2 136.688637 225 Nb2O5 104.346797226 SiO2 156.347975 227 Nb2O5 75.50798 228 SiO2 131.274802 229 Nb2O5106.072884 230 SiO2 161.095229 231 Nb2O5 73.862257 232 SiO2 123.798118233 Nb2O5 120.45028 234 SiO2 139.021448 235 Nb2O5 61.510898 236 SiO2186.49687 237 Nb2O5 110.855316 238 SiO2 95.411626 239 Nb2O5 112.998509240 SiO2 85.040064

Like the above examples, the design for the filter of example 8 wasproduced by optimizing a standard dichroic filter comprising alternatingquarter wavelength thick layers of SiO₂ and Nb₂O₅. However, in thiscase, the basic structure and spectra were optimized against the targetspectra 173, 181, 183, and 185 shown in FIGS. 16 and 17.

The calculated spectrum of this design exhibited a fundamental stopbandregion having a short wave edge around 690 nm and a long wave edgearound 850 nm. The calculated spectrum also exhibited a non-harmonicstopband having a long wave edge around 540 nm, and a shortwave edgearound 520 nm. A passband region separated the fundamental andnon-harmonic stopband regions, and had a passband bandwidth of 23.5%,relative to the long wave edge of the non-harmonic stopband. The edgesteepness (10-90% T of the long wave edge of the non-harmonic stopbandwas 0.10%. Finally, the calculated spectrum shows that the designsubstantially blocks 100% of light (OD 6 or greater) having a wavelengthwithin the fundamental and non-harmonic stopband regions.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1-11. (canceled)
 12. An interference filter, comprising: a substrate,and a plurality of alternating first and second material layersdeposited on said substrate, said material layers having respectivelydifferent refractive indices; wherein said plurality of alternatingfirst and second material layers are configured such that when lightfrom a light source impinges on said filter at an Angle of incidence ofabout 45°, said filter defines a spectrum for light of averagepolarization, said spectrum comprising a first stopband region and asecond stopband region separated by a passband region having a passbandbandwidth; wherein said first stopband region corresponds to afundamental stopband; wherein said second stopband region comprises along wave edge and a short wave edge, and encompasses a range ofwavelengths other than wavelengths corresponding to a harmonic of thefirst stopband region; and wherein at least one of said long wave edgeand short wave edge has an edge steepness, when measured between 10-90%transmission, of about 0.75% or less, relative to an edges wavelength ofsaid second stopband region.
 13. The interference filter of claim 12,wherein at least one of said long wave edge and short wave edge exhibitsan edge steepness, when measured from optical density 3 to 90%transmission, of about 0.50% or less.
 14. The interference filter ofclaim 12, wherein at least one of said long wave edge and short waveedge exhibits an edge steepness, when measured from optical density 1 to90% transmission, of about 0.25% or less. 15-18. (canceled)